1. Field of the Invention The present invention relates to a method for managing a pre-established session, a Push-To-Talk (PTT) over Cellular (PoC) system and PoC user equipment (UE) for implementing the same. More particularly, the present invention relates to a method for enabling a pre-established session to be maintained when a PoC client in a terminating side opens a PoC session using the pre-established session and terminates the open PoC session, a PoC system and a PoC UE for implementing the same.
2. Description of the Related Art
Significant development of mobile communication technologies and the expansion of communication networks have led to the provision of various services and applications which use mobile telephones. In addition to basic communication services, users are demanding more diversified services, including a positioning service, a multimedia service, and a Push-to-Talk (PTT) service. In particular, the PTT service supports various supplementary functions, such as instant messenger, status display, and a voice/multimedia storing service, as well as group and voice communications which have been provided by conventional radio transmitters or trunked radio systems (TRSs).
Currently, various efforts are being made in the industry to standardize a PTT-over-Cellular (PoC) service which employs a PTT function in a mobile communication network. One feature of the PoC service, which draws a distinction between the PoC service and the conventional mobile communication service, is that the user can perform communications while moving between sessions, if necessary, because he/she belongs to a plurality of sessions. The requirement that the user must be able to communicate while moving through a plurality of PoC sessions is specified by the Open Mobile Alliance (OMA), which is an organization regulating pertinent mobile communication services.
Meanwhile, in order to make use of the PoC call service, a PoC client registers his/her PoC address with a Session Initiation Protocol (SIP)/Internet protocol (IP) core. Then, the SIP/IP core stores information about the PoC client according to the request of the PoC client. Thus, when another PoC client tries to request a PoC group call, the PoC client registers his/her information in the SIP/IP core network in advance, as described above, and requests the PoC group call to his/her SIP/IP core network by using group identification information received from a PoC Extensible Markup Language (XML) Document Management Server (XDMS). Then, the SIP/IP core performs address determination and domain location determination procedures by using information on the PoC client which has transmitted the call request, and then transfers the PoC call request to a home PoC server with which the call requesting PoC client has been registered. The PoC server prepares to open a PoC session in response to the PoC communication request, acquires the requisite client information from the PoC XDMS server, and transfers communication request signals to the corresponding SIP/IP core network. In this case, when the communication request signal corresponds to a communication request for clients within an intradomain, the PoC server performs both a participating PoC function (PF) and a controlling PoC function (CF).
The PoC server, which manages a call-requested PoC client, requests a PoC call to the PoC client, after the SIP/IP core network performs the location determination procedure, by using information about the PoC client that is received by the PoC server. When the PoC-call-requested PoC sends an OK response to the PoC client who has requested the call, a PoC communication begins. When PoC communication is not started due to the status of a counterpart PoC client, the PoC client can store the user's voice and/or media to be transmitted in a PoC box.
Meanwhile, call processing technology for PoC communication establishment, which makes communication possible according to a communication request as described above, may have various procedures depending on requirements and statuses of originating and terminating sides. Required characteristics of a PoC system according to the requirements and statuses of the originating and terminating sides, based on the OMA, are as follows:
First, the terminating side can establish its own answer mode according to the request of a PoC client, in which the answer mode may be classified into an auto-answer mode and a manual answer mode.
In the auto-answer mode, the answer is immediately transmitted from an associated network to the transmitting side instead of the receiver being manually answered when the originating side is included in a PoC client list designated by the terminating-side PoC user. The network can automatically send the answer on behalf of the UE, as described above, because the PoC server has a function of storing an answer mode and an associated user list according to an answer-mode setting request of the UE. Meanwhile, the manual answer mode is used when it is unclear who an associated user is or if the user is not included in the user list corresponding to the automatic answer mode, or when the receiver sets the manual answer mode for all users, in which a PoC communication request is transmitted to the UE through a reception network and the communication is connected after the PoC client's permission.
Second, the PoC system classifies session modes into on-demand and pre-established session modes according to whether a connection has been established between the PoC client and the PoC server belonging to the user's home network.
The on-demand session mode corresponds to when no pre-established session has been established by a PoC client, in which the PoC client performs a procedure for connecting a PoC call after receiving an invitation message from another PoC client.
Meanwhile, in the pre-established session mode, a specified session is pre-established by the PoC client between the PoC client and the PoC server belonging to the user's home network, according to the request of the PoC client. Such a pre-established session is necessary to negotiate media parameters to be used by the PoC client with the PoC server in advance so that a session can be quickly established without renegotiation for the media parameters to be used between the PoC server and the PoC client in the future. For setting a pre-established session, a PoC client provides media parameters supportable through a Session Description. Protocol (SDP) body using a SIP INVITE method, and responds to media parameters provided from the server, in which a response message from the server includes identification information of a newly pre-established session and a conference uniform resource identifier (URI). When such a pre-established session is used, it is possible to pre-establish an IP address, a port number, a codec to be used, and a talk burst control protocol. That is, in the PoC service, a pre-established session is created and maintained to rapidly open a PoC session. When a pre-established session is opened, parameters, such as an IP address, a port number, a codec, etc., are negotiated between a PoC client and a PoC server in advance. Accordingly, when a response is received after a REFER message has been sent to open a PoC session, or when a response is sent after an INVITE message has been received, the parameters are used to open a PoC session, so that it is possible to reduce a period of time required for establishment of the session.
A conventional signal flow for establishing a pre-established session between a PoC client and a PoC server will now be described with reference to FIG. 1.
In steps 100 and 102, PoC client A transmits an INVITE message, which is a message for the establishment of a pre-established session, to PoC server A through SIP/IP core A. Thereafter, in steps 104 and 106 the PoC server A transmits an OK message to the PoC client A through SIP/IP core A in response to the INVITE message.
A conventional signal flow for opening a PoC session between the PoC client and the PoC server in the state in which the pre-established session has been established as described above will now be described with reference to FIG. 2.
When the PoC client A is to open a PoC session by using the pre-established session, the PoC client transmits a REFER message for opening the PoC session to the PoC server A through the SIP/IP core A in steps 200 and 202. In this case, the PoC client A opens the PoC session with the pre-established session that has been maintained, that is, by using a Call ID of an SIP session between the PoC client A and a PoC server (i.e., a participating PoC function unit), which performs a participant management function, as a pre-established session ID.
Thereafter, the PoC server performs an operation for inviting PoC clients invited to the corresponding PoC session in step 203, and transmits an ACCEPTED message to the PoC client A through the SIP/IP core A in response to the REFER message in steps 204 and 206.
Thereafter, when receiving a first invitation permission response from a specific PoC client in step 208, the PoC server A transfers a PoC session ID to the PoC client A through a floor connection (i.e. Talk Burst connection) message in step 209.
Then, the PoC client A transmits a floor acknowledgement (i.e. Talk Burst Acknowledge) message to the PoC server A in step 210, and the PoC server A transmits a floor confirmation (i.e. Talk Burst Confirm) message to the PoC client A in step 212. Thereafter, the PoC client A transmits Real-time Transport Protocol (RTP) media to the PoC server A in step 214.
Meanwhile, when the PoC server A receives a response to the invitation from another PoC client in step 215, the PoC server A transmits a NOTIFY message to the PoC client A through the SIP/IP core A in steps 216 and 218. Then, the PoC client A, having received the NOTIFY message, transmits an OK message to the PoC server A through the SIP/IP core A in steps 220 and 222.
Hereinafter, a conventional signal flow for a procedure of terminating a PoC session with a maintained pre-established session when both an originating side and a terminating side are in the auto-answer mode will be described with reference to FIG. 3.
First, when PoC client A is to terminate a PoC session in a state in which the PoC session has been established as shown in step 300, the PoC client A transmits a REFER BYE message, which requests the PoC session to be terminated, to PoC server A in steps 302 and 304. In this case, the PoC client A stores an ID and a media parameter of a PoC server of a participating PoC function (PF) and an ID and a media parameter of a PoC server of a controlling PoC function (CF). Therefore, the PoC client A transmits a REFER BYE message using the two IDs, so that the ciphering can terminate the PoC session with the pre-established session that has been maintained.
Then, the PoC server A transmits an ACCEPTED message in response to the REFER BYE message to the PoC client A in steps 306 and 307.
Thereafter, the PoC server A transmits a BYE message, which requests a session to be terminated, through SIP/IP cores to PoC server X in steps 310, 312 and 314. The PoC server X, which has received the BYE message, transmits an OK message through the SIP/IP cores to the PoC server A in response to the BYE message in steps 316, 318 and 320. Thereafter, the PoC server A transmits a NOTIFY message through the SIP/IP core A to the PoC client A in steps 322 and 324, and the PoC client A, having received the NOTIFY message, transmits an OK response message through the SIP/IP core A to the PoC server A in steps 326 and 328.
Then, the PoC server A terminates the PoC session, while maintaining the pre-established session in steps 330, 332 and 333.
That is, the conventional method for terminating a PoC session with a pre-established session that has been maintained can be only performed by an originating side.
Hereinafter, a conventional signal flow for the opening of a PoC session between a PoC client and a PoC server when a terminating side is in a manual answer mode will be described with reference to FIG. 4. First, it is assumed that a pre-established session has been established between PoC server B (PF entity) and terminating-side PoC client B. When receiving an INVITE message from PoC server A (CF entity) having a session control function in step 400, the PoC server B (PF entity) transmits a re-INVITE message to the terminating-side PoC client B in step 402. In this case, when the PoC server B (PF entity) performing a participant management function transmits the re-INVITE message, the PoC server B inserts a PoC session ID into conference URI information, thereby changing a PoC session contact address of the PoC client from an existing pre-established session ID to the PoC session ID.
The PoC client B, having received the re-INVITE message, transmits an alerting message to the PoC server A through the PoC server B in steps 404 and 406. In this case, since the PoC client B has received the re-INVITE message including the PoC session ID in place of the existing pre-established session ID, the PoC client B cannot have information about a pre-established session ID any more. In addition, the terminating-side PoC client B transmits a 200 OK message to the PoC server A (CF entity) in steps 408 and 410.
As described above, in the case where a terminating-side PoC client is in the manual answer mode using the pre-established session, when the PoC server B (PF entity) performing the participant management function in the terminating side transmits a re-INVITE message, the PoC server B inserts a PoC session ID into a Contact header according to an SIP mechanism defined in the SIP mechanism RFC [3261], so that the PoC session contact address of the PoC client is changed from an existing pre-established session ID to a PoC session ID. The entire disclosure of RFC [3261] is hereby incorporated by reference. Accordingly, the PoC client will no longer have information about the pre-established session ID.